1. Field of the Invention
The present invention relates to an X-ray mask for manufacturing semiconductor devices such as IC and ULSI. More particularly it is concerned with an X-ray mask suited to an exposure apparatus for manufacturing semiconductor devices, that uses a soft X-ray having wavelengths of about 2 .ANG. to about 150 .ANG..
2. Related Background Art
In recent years, in exposure apparatus for manufacturing semiconductor devices such as IC and LSI, many kinds of exposure apparatus utilizing soft X-rays and capable of obtaining prints with a higher resolution have been proposed as the semiconductor devices have become more highly integrated.
In general, the X-ray mask used in this soft-X-ray apparatus includes a support frame of ring shape, and a membraneous member stretchingly covering the support frame opening and having X-ray transmitting portions and X-ray non-transmitting portions.
The non-transmitting portions are formed of an X-ray opaque material (including an absorber) having a geometrical pattern and are provided on a support membrane (a mask membrane) that constitute a substrate in the membraneous member. The X-ray opaque material is so arranged that patterns are accurately transferred onto a wafer surface in the size on the order of submicrons. On the other hand, the transmitting portions are made of the mask support membrane itself corresponding to the part on which the light-shielding material is not provided.
In the exposure apparatus using soft X-rays, both the space in which the X-ray mask is disposed and the upper space of a wafer on the side on which a resist is coated are, in many instances, put under a reduced pressure atmosphere or a low-pressure helium atmosphere in order to prevent absorption loss of energy of the light irradiated from a soft X-ray source for exposure.
If exposure is carried out using soft X-rays having wavelengths, for example, of approximately from 2 to 150 .ANG. under such conditions, the X-ray mask support membrane, a mask absorber and atmospheric gas atoms absorb the soft X-ray and emit photoelectrons by virtue of a photoelectric effect. In particular, when the support membrane includes an insulator thin membrane, such exposure causes not only: electrostatic charging due to the emission of photoelectrons from the constituent atoms of the support membrane, but also electrostatic charging due to the emission of photoelectrons from the absorber having a large number of electrons, resulting in a support membrane having a high positive potential.
In general, the distance between the support membrane and the resist surface on the wafer is so small that run-out error caused by divergent light from the source can be reduced to a minium, and is set, for example, in the range of from 10 .mu.m to 100 .mu.m. For this reason, the electrically charged support membrane and wafer may electrostatically attract each other resulting in deformation of the support membrane. Thus, precision of the exposed pattern is sometimes lowered, because of flexure of the support membrane or because the support membrane contacts the wafer when the attraction force is strong.
In an exposure apparatus attempting to obtain a high resolution such that the size of a pattern to be transferred is 0.5 .mu.m or less, the support membrane is commonly so constituted that it may have a thickness of about 2 .mu.m, using an inorganic material (ceramics in particular) having a small thermal expansion coefficient and large Young's modulus so that misregistration due to its thermal expansion or the distortion of absorbers due to residual stress can be suppressed.
In general, many of the materials for this purpose are brittle and hard. Hence, it has sometimes occurred that the support membrane was easily broken by excessive deformation or application of nonuniform pressure.
In order to overcome this problem, there is, for example, a method in which a metal having reduced absorption to X-rays, as exemplified by aluminum, is deposited in a thickness on the order of hundreds of .ANG. on the surface. This method can achieve a good electric conductance and good soft-X-ray transmittance, but has involved problems that the transmission of visible or infrared light in making alignment with the wafer is so extremely low that the precision of alignment may be extremely lowered.